The invention has been devised primarily (but not exclusively) in connection with optical printers used to make duplicate prints of a film from a masters. An optical printer typically includes at least one projection head through which the master film is indexed frame by frame, a camera loaded with unexposed film onto which the frames are to be copied, and a light source for projecting successive frames of the master onto the film in the camera, through suitable lenses. The projection head includes a film transport mechanism which brings successive frames of the master into registration with an aperture for projection. The film in the camera is usually advanced in synchronism with the film in the projection head.
For accurate duplication of the images on the master film, it is important that the film be accurately located for projection of each frame, not only laterally with respect to the aperture but also along the projection axis (i.e. towards and away from the aperture). The film should lie flat precisely in the focal plane of the projection lens if the image is to be sharply in focus and precisely the correct size on the film in the camera. This is particularly important in the case of large format films such as those used with IMAX (registered trade mark) film projection systems.